In recent years, the oil well drilling environment has become more and more severe, and steel pipes for oil wells used on each spot are now exposed to an oil well drilling environment containing carbon dioxide and the like in addition to the increasing depth of oil wells. The steel material to be used in producing such steel pipes is required to have strength and toughness characteristics. In particular, oil wells to be developed in the future are expected to be ones having a greater depth or horizontal ones and, therefore, the steel pipes to be used are required to have still higher strength and toughness performance characteristics than the levels so far required.
To cope with these requirements, the art has endeavored to produce high performance steel pipes by reducing the size of austenite grains in the steel material or by adding an expensive additive element or elements to thereby improve the hardenabihty. From such a viewpoint, Japanese Patent No. 2672441, for instance, proposes a method of producing seamless steel pipes characterized by high strength and high toughness.
According to the production method proposed in the above-cited patent specification, the austenite grain size is reduced to ASTM No. 9 or finer to thereby secure excellent resistance to sulfide stress corrosion cracking (SSCC resistance) as well as high strength and toughness performance characteristics.
Thus, the production method proposed in the above patent specification is intended to give steel species having high toughness and employs the so far known technique of reducing the size of austenite grains and, therefore, it is expected that the reduction in size of austenite grains will cause deterioration in hardenability. When the hardenability of a steel species becomes poor, the toughness and corrosion resistance will deteriorate. For preventing the hardenability of steel from deteriorating, it is generally necessary to add a large amount of such an expensive element or elements as Mo.
Furthermore, the production method proposed in the above-cited patent specification presupposes that direct quenching or in-line heat treatment be performed directly from the heated state after rolling, which is then followed by tempering. Therefore, the method requires strict control of rolling conditions and, in this respect, it is unsatisfactory for the cost rationalization and production efficiency viewpoint. The method still has the problem that the productivity improvement, energy saving and cost reduction currently required in the production of steel pipes for oil wells cannot be accomplished.
On the other hand, methods of producing steel pipes for oil wells capable of showing good performance characteristics in oil well environments even when the size of austenite grains is relatively coarse have been proposed. Since intragranular cracking serves as the origin of breakage with the increasing strength of steel, Japanese Patent Application Laid-open No. S58-224116, for instance, proposes a method of producing seamless steel pipes excellent in sulfide stress cracking resistance which comprises reducing the contents of P, S and Mn, adding Mo and Nb, and controlling the austenite grain size within the range of 4 to 8.5.
Further, Japanese Patent No. 2579094 proposes a method of producing oil well steel pipes having high strength and excellent sulfide stress corrosion cracking resistance which comprises adjusting the steel composition and hot rolling conditions to thereby adjust the austenite grain size to 6.3 to 7.3.
However, any of the methods so far proposed does not mention anything about the securing of toughness required of steel pipes for oil wells and cannot be employed as a method of producing oil well steel pipes having both high strength and high toughness.
Meanwhile, it is known that, for securing the toughness of steel materials, it is effective to strengthen the austenite grain boundaries themselves in place of reducing the austenite grain size. As a means therefor, a method is known which comprises controlling the carbides precipitating on austenite grain boundaries. Thus, grain boundaries, as compared with intragranular, are the places where carbides tends to readily precipitate and where carbides readily condense, so that the strength of grain boundaries itself tends to decrease.
Therefore, it becomes possible to improve the toughness of steel materials when coarse carbide precipitation and/or carbide condensation at austenite grain boundaries is prevented. For such reasons, high levels of toughness cannot be attained without controlling the carbides precipitating on grain boundaries when the austenite grains are relatively coarse as with the steel species disclosed in the above-cited Japanese Patent Application Laid-open No. S58-224116 and Japanese Patent No. 2579094.
From such viewpoints, methods of inhibiting the precipitation of carbides which tend to become coarse at austenite grain boundaries have recently attracted attention. Among carbides which may occur in low alloy steel species containing Cr and Mo, there are the types M3C, M7C3, M23C6, M3C and MC. Among these, carbides of the M23C6 type are thermodynamically stable and readily precipitate and, at the same time, are coarse carbides, so that they decrease the toughness of steel materials. Further, M3C type carbides are acicular in shape and increase the stress concentration coefficient, hence decrease the SSCC resistance.
For the reasons mentioned above, methods have now been proposed for inhibiting the precipitation of M23C6 type and/or M3C type carbides. For example, Japanese Patent Application Laid-open No. 2000-178682, Japanese Patent Application Laid-open No. 2000-256783, Japanese Patent Application Laid-open No.2000-297344, Japanese Patent Application Laid-open No. 2000-17389 and Japanese Patent Application Laid-open No.2001-73086 disclose steel species or steel pipes with reduced contents of M23C6 type carbides. However, the methods disclosed in these publications pay attention only to the controlling of M23C6 type carbides but do not take into consideration the influences of the austenite grain size; therefore, it must be said that the hardenability of steel is sacrificed in them.
In other words, under the circumstances, none of the methods relying only on the technique of reducing the austenite grain size or only on the technique of controlling carbides tending to become coarse can accomplish the intended objects in producing steel species or steel pipes having high strength and high toughness and excellent sulfide stress corrosion cracking resistance (SSCC resistance) at low cost. Therefore, guidelines are desired for optimally combining and for making good use of both the effect of carbide control and the effect of reducing the austenite grain size so that steel species or steel pipes suited for use in oil well environments can be produced at low cost.